The curling of print media sheets is a particular problem in the printing industry, and is exacerbated by high-density images and plural color printing. Sheet curling, however, can occur even in the context of unprinted sheets of paper due to changes in ambient humidity or moisture content of the paper. Sheet curling can interfere with proper sheet feeding, causing sheet feeding jams or delays. If sheet curl is present in the output, it can interfere with proper stacking or other finishing operations. For example, if printed sheets with curl do not lie flat when stacked together in sets, such as in the pages of booklets, an objectionable distortion of the resulting booklet may result.
Furthermore, the amount of moisture in the sheet of paper can drastically change from the printing process itself, to cause or exacerbate curl. In particular, from water-based ink jet printing or the thermal fusing operation for toners in xerographic printing, and particular from high density image printing near the edges of the sheet. There is a further sheet curl problem in duplex printing, where the sheets are re-fed or recirculated for printing imaging material on their second sides, especially if that involves a second pass of the sheet through a thermal fuser and/or higher density images on one side than the other.
In order to control or remove the amount of curl, the print media curl must be measured. Various paper curl sensors and control apparatus are known in the electrophotographic printing arts. One example is disclosed U.S. Pat. No. 6,668,155, entitled “Lead Edge Paper Curl Sensor,” which issued to Hubble, III, et al. on Dec. 23, 2003 and is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 6,668,155, which is incorporated herein by reference in its entirety, discloses a sheet curl sensor that remotely senses sheet curl without contacting or interfering with the motion of the sheets in their normal sheet path. This sensor operates on a portion of the moving sheet at an angle thereto and perpendicular thereto, with displacement insensitive optics, in both an angular direction substantially parallel to the sheet movement direction and an angular direction substantially transverse to the sheet movement direction, with rationing of the two input signals. In such a sheet curl sensor, the variable output control signals in response to the sensed illumination are a ratio of the output control signals from the photodetector system produced by the first and second illuminators. The ratio of the output signals from the photodetector system is then proportional to the amount of the paper curl sensed.
Another example of a sheet curl sensor is disclosed in U.S. Pat. No. 5,270,778, entitled “Sheet Curl Control Apparatus,” which issued to Andrew Wyer on Dec. 14, 1993 and is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 5,270,778, which is incorporated herein by reference in its entirety, discloses a sheet curl sensor comprising a radiation source, in the form of an infra-red emitter and two detectors in the form of infra-red sensors. The sensors are spaced apart adjacent a horizontal section of sheet path and are arranged whereby movement of the sheet material along the sheet path causes the infra-red light beams to be interrupted in succession by the lead edge of the sheet material. The time interval between interruptions of the infra-red light beams at the sensors is a function of the sheet curl.
U.S. Pat. No. 5,751,443, entitled “Adaptive Sensor and Interface,” which issued to Borton et al on May 12, 1998 and assigned to the Xerox Corporation is an example of a precise lead edge sensing system. U.S. Pat. No. 5,751,443, which is incorporated herein by reference in its entirety, discloses a sensor which detects the presence of paper and transparencies in a sheet transporting path and includes a light source disposed near the transporting path for projecting light toward a reflector on the opposite side of the transporting path and a light detector located relative to the light source to receive light emitted by the light source and reflected b the reflector so that by such positioning the light path is interrupted by substrates passing through the transport path. The output signal of the light detector is proportional to the light received across the transport path. A control, electrically connected to the sensor, adjusts flux incident on the light detector to maintain the collector current in the linear portion of the light detector's operating range. The sensor is tilted at an angle with respect to the horizontal of a copy substrate to be able to detect transparencies.
One problem encountered by prior art sheet curl sensors relates to the maximum resolution of the sensor. Expensive and complicated solutions have been used to measure to a resolution of less that 0.01 mm. The ability to properly measure and accurately control the lead edge sheet curl depends on the proper constraint of the leading edge of the sheet. A single elastomer roller is inadequate since the idler compresses the elastomer roller and the exit angle of the sheet therefore depends on the elastomer properties, the roller forces and the sheet media stiffness and weight. A constraint system is therefore required, which will not damage or mark the media sheets while ensuring that the sheets remain at a consistent height and angle.